An estimated 11.1 million illegal immigrants lived across the United States as of March 2009, down nearly 8 percent from a peak of 12 million in 2007. Still, the unauthorized population remains a third larger than it was in 2000.
"This is something we haven't seen before," Jeffrey Passel, a senior Pew demographer, said of the decline. "It's a rather large decline and sustained over two years."
Passel attributed the new trend to a sharp decrease in the number of illegal immigrants crossing U.S. borders in recent years.
The in-flow of unauthorized immigrants averaged 500,000 per year for the first half of the decade, but plummeted to an estimated 150,000 a year between 2007 and 2009, according to the report.
"There is constant in and out movement among immigrants," Passel said. "But what's changed most is the number coming in."
While the study does not identify reasons for the decline, experts say stepped up enforcement of immigration laws and enhanced border security measures have contributed to the change.
Over the past decade, lawmakers have approved funding for more border patrol agents, construction of hundreds of miles of border "fence," deployment of unmanned aerial drones and new systems for employers to verify the legal status of immigrant workers.
Immigration and Customs Enforcement has detained and deported an increasing number of illegal immigrants since 2001, removing 387,000 last year alone -- a seventh consecutive record high.
But the U.S. economic downturn has also played a key role in mitigating the incentive for prospective illegal migrants, who typically come in search of money and jobs.
"Because it's harder to get in and when you get in you may not be able to find a job, fewer appear to be trying," Passel said.
The national unemployment rate was 9 percent in March 2009, when the data for the Pew study was collected. But the rate exceeded 10 percent among unauthorized immigrant workers.
"How bad does the economy have to get for us to fix the immigration system," said Ali Noorani, executive director of the National Immigration Forum. "If five percent of our work force is undocumented, that means unscrupulous employers are able to push down the wages of those workers and that means lower wages for everyone else."
The Pew study found that 5 percent of the country's labor force, or 7.8 million people, are illegal immigrants, a number that has held relatively steady in recent years.
States with the largest shares of illegal immigrant workers are Nevada, California, Texas and New Jersey.